IMO if you're going to want to be waterproof, you'll want to remain brushed. If you do go brushless, you can not run a sensored motor, as they're not waterproof. Brushed motors are inexpensive, have all the power you need, and are waterproof. I run my Wraith in water, snow, mud, etc and no issues with my motor.

If you're looking to build a crawler, 3S is a lot of juice. You'll get a lot of wheel spin with 3S, so you might want to consider running 2S. See what guys are running in the crawler forums to compare, but I would think 3S in an SCX10 would be too much juice for crawling.

As for running BEC, it's not a "must have" by any means. I'm running an RTR Wraith Poison Spyder with the supplied Castle ESC and it works great. Plasti-dipped it to waterproof it, and she works like a charm!

If you have any questions (or you're ever in Edmonton and want to hit some trails) let me know!